Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to a technology for correcting periodical density fluctuations that occur in the sub-scan direction.
Description of the Related Art
In an image forming apparatus (such as a printer, a copying machine, or a facsimile machine) using an electrophotographic process technology, an electrostatic latent image is normally formed on the surface of a photosensitive member (a photosensitive drum, for example) when the photosensitive member that is uniformly charged is illuminated with (exposed to) light based on input image data. Toner is then applied onto the photosensitive member having the electrostatic latent image formed thereon, so that the electrostatic latent image is visualized to form a toner image. After transferred directly onto a paper sheet or indirectly onto a paper sheet via an intermediate transfer member, this toner image is heated and pressed by a fixing unit, to form an image on the paper sheet.
An image forming apparatus includes rotating members such as photosensitive members and developer carriers as the components for image formation. It is known that periodical density fluctuations in the image sub-scan direction occur due to rotational deflection of those rotating members. For example, the distance (a development gap) between a photosensitive member and a developer carrier periodically changes due to rotational deflection of the photosensitive member or the developer carrier. Because of this, the field intensity periodically varies, even when a constant developing bias is applied. As a result, density fluctuations occur in images in the same cycles as the rotation cycles of the photosensitive member or the developer carrier. Hereinafter, periodical density fluctuations that occur in the image sub-scan direction will be referred to as “periodical density unevenness”.
In a conventional image forming apparatus, correction data corresponding to the rotation position (the phase based on the home position) of a photosensitive member is created in accordance with a density profile indicating periodical density unevenness, so that the periodical density unevenness can be eliminated. With this correction data, image forming conditions such as the exposure energy (exposure time or exposure power), the charging voltage, the developing bias voltage, and the number of rotations of a developer carrier (a developing roller, for example), and the density value (tone value) of input image data are corrected (see JP 2014-219453 A).
A density profile is created by forming a density correction patch image (a halftone image having a halftone density, for example) on a toner image carrier such as an intermediate transfer belt, and detecting the image density in the correction patch image. This correction patch image is formed so that the length in the sub-scan direction becomes greater than the greatest cycle length (normally, the cycle length of a photosensitive member) among the cycle lengths (equivalent to the rotation periods) of rotating members that might cause periodical density unevenness. Alternatively, a correction patch image that is longer than a multiple of the cycle length of a rotating member is formed, and the mean value of detected image densities is calculated. In this manner, a high-precision density profile can be obtained.
To increase density correction accuracy, the density profile is preferably updated regularly or at a predetermined time such as the start of a print job. This is because the density profile changes as the development and transfer characteristics change with environments or with the passage of time.
If the length of the correction patch image in the sub-scan direction is increased, or if the density profile is frequently updated, density correction accuracy is increased, but the following problems are caused: the load to be imposed on the cleaning unit when the correction patch image formed on the toner image carrier is removed becomes larger; the amount of toner to be consumed in the density correction becomes larger; and a longer period of time is required for the density correction, resulting in a decrease in productivity, for example.